The invention pertains to a solid-state color imager, specifically, such an imager of the MOS FET type.
An example of a prior art approach to an imager of the same general type to which the invention pertains is described in an article "2/3-Inch Format MOS Single-Chip Color Imager", IEEE Transactions on Electron Devices, Vol. ED-29, No. 4, April 1982 by M. Aoki et al. The imager described in this article, and as also illustrated herein in FIGS. 1 and 2, includes a sensor array 100 (FIG. 1) composed of a large number of pixels (picture elements) arranged in a matrix of rows and columns. The sensor array 100 includes horizontally extending electrodes 21, 22, 23, 24, . . . which intersect with vertically extending electrodes 30, 31, 32, 33 . . . All of these electrodes are electrically insulated from one another. Each pixel 10 includes a light sensing element, composed of a photodiode 12 and a parallel-connected capacitor 13, and a switching element, implemented as an FET device 11. The gate 15 of the FET device 11 is connected to a corresponding horizontally extending electrode (electrode 21 for the pixel 10 in the upper left-hand portion of FIG. 2), the drain of the FET device 11 is connected to a respective one of the vertically extending electrodes (electrode 30 for the pixel 10 in the upper left-hand portion of FIG. 2), and the source of the FET device 11 is coupled to the cathode of the photodiode 12. Color filters are provided over each of the photodiodes 12, with the filters being arranged in an appropriate pattern for color imaging. In the example of FIG. 2, "W" indicates the presence of a white filter over the respective photodiode, "Ye" indicates a yellow filter, "G" indicates a green filter, and "Cy" indicates a cyan filter. As is well known to those of ordinary skill in the art, other filter patterns can be used as well.
The vertically extending electrodes (hereinafter termed "horizontal lines" for reasons which will later become clear) 30, 31, 32, 33 . . . are each associated with an output signal which carries information about a single color. Specifically, in FIG. 2, H.sub.w (j), H.sub.G (j), H.sub.Ye (j) and H.sub.Cy (j) indicate the j-th white, green, yellow and cyan output signals, respectively. To effect this arrangement, each of the lines 30, 31, 32, 33 . . . is connected to the drains of all FET devices of all pixels of the associated color in the corresponding column. For instance, the line 31 is connected to all drains of all FET devices 15 in the corresponding column which are coupled to photodiodes 12 having green filters.
The horizontally extending conductors (hereinafter termed "vertical lines", again for reasons which will later become clear) 21, 22, 23, 24 . . . each connect to the gates 15 of all FET devices 11 of all pixels in the corresponding row.
During the operation of the array, between readout times, the conductivity of the photodiodes 12 varies in accordance with the intensity of the light shone thereon. Accordingly, the charge across each of the capacitors 13 at the end of the read-out period is determined by the average intensity of the light which shone on the corresponding photodiodes 12 between read-out times. To read out the color information signals thus generated from each of the pixels 10, the vertical lines 21, 22, 23, 24 . . . are activated, that is, raised to a positive voltage, in a particular sequence. Because the activation of the vertical lines 21, 22, 23, 24 . . . in sequence corresponds to scanning the sensor array in the vertical direction, the lines 21, 22, 23, 24 are termed a "vertical lines", even though each line extends in the horizontal direction.
FIGS. 4A and 4B show a segment of an integrated circuit implementation of the circuit of FIG. 2. The vertical lines 30 and 31 are arranged intersecting, but insulated from, the horizontal lines 21 and 22. A light sensing area is formed in the rectangular openings defined between intersecting pairs of the horizontal and vertical lines, for instance, between lines 21, 22, 30 and 31. Source diffusions 19 (indicated by dash-dot-dash lines) are formed in the substrate 41, extending into each of the openings. A diode-junction forming layer (not shown) is provided over portions of each source diffusion to form the photodiodes. Other portions of each of the source regions 19 extend under respective ones of the horizontal lines, for instance, a portion of the source diffusion 19 which is partially within the opening bounded by the lines 21, 22, 30 and 31, shown in the center of FIG. 4A, extends under the horizontal line 30. The source diffusion 19 extends up to a position under and adjacent to a side of a respective one of the vertical lines. For instance, the source diffusion 19 in the center of FIG. 4A extends up to the lower edge of the line 21. A source contact 17 is thusly formed. Drain diffusions 18 (indicated by dash-dot-dot-dash lines), which make electrical contact with respective ones of the horizontal lines at drain contact areas 16, also extend under corresponding horizontal lines, up to positions adjacent to but under the edge of the respective vertical line and opposite the corresponding source diffusion 19. At intersections between the horizontal and vertical lines where the source diffusion 19 and drain diffusion 18 are formed, the vertical lines 21, 22, etc. are separated from the substrate only by a thin oxide layer. In this manner, a gate region 15 is formed between each drain diffusion 18 and source diffusion 19. For intersections of vertical and horizontal lines where the drain and source diffusions do not extend up to the edges of the vertical line, a thick insulating layer is provided to separate the vertical line from the surface of the substrate so as to prevent the potential on the vertical line from interfering with the operation of the device. This arrangement is illustrated in the cross-sectional view of FIG. 4B where the vertical line 21 is separated from the surface of the substrate 41 by a layer of field oxide 40, while the line 22 is separated from the surface of the substrate 41 by only a thin oxide layer between the source diffusion 19 and drain diffusion 18.
The circuitry used for reading out the color information signals from the pixels 10 and communicating these signals to the utilization circuitry (display device, recording device, etc.) is depicted in FIG. 1. To activate the vertical lines 21, 22, 23, 24 . . . a vertical shift register 103 is provided. The vertical shift register successively and sequentially sets a positive voltage on its output lines VS(O), . . . , VS(i), . . . VS(n-1), VS(n) which are applied to an interlace circuit 104. The interlace circuit 104 also receives signals FA (Frame A) and FB (Frame B) indicative of which of two interlaced frames is currently to be scanned. The outputs of the interface circuit 104 are buffered and applied to the vertical lines 21, 22, 23, 24, . . . of the sensor array 100 by a buffer circuit 105 with the aid of clock signals .phi..sub.V2 and .phi..sub.V3. The outputs from the sensor array H.sub.W (j), H.sub.G (j), etc., on lines 30, 31, etc., are applied to an array output switching circuit 102 and communicated upon a bus 107 to the utilization circuitry in accordance with outputs provided by a horizontal shift register 101.
In the sensor array shown in FIG. 2, to provide the color information necessary to form a complete picture element, it is necessary to read out the color signal information contained in four adjacent pixels, namely, a block of adjacent white, yellow, green and cyan sensitive pixels, and hence to cause color signal read out on four horizontal lines (e.g., H.sub.w (j), H.sub.G (j), H.sub.Ye (j), H.sub.Cy (j)). Moreover, luminance information is provided with every vertical pair (e.g., H.sub.w (j), H.sub.G (j) and H.sub.Ye (j), H.sub.Cy (j)), thus providing improved horizontal resolution. To read out the color information, it is necessary to activate two of the vertical lines 21, 22, 23, 24, . . . for each line scan, that is, for each of VS(0), etc. Further, for purposes of interlacing, it is necessary that the pairing of the vertical lines be changed between frames. For instance, for a frame A, the lines 21 and 22 are simultaneously activated, followed by simultaneous activation of lines 23 and 24. For the subsequent frame B, the lines 22 and 23 are simultaneously activated, followed by simultaneous activation of line 24 and a next following line (not shown). This is indicated at the left-hand side of FIG. 2 by signals V.sub.A (i) (vertical line, Frame A, i-th line scan), V.sub.A (i+1), . . . V.sub.B (i), V.sub.B (i+1). For interlaced scanning, the activation sequence is . . . V.sub.A (0), . . . , V.sub.A (i), V.sub.A (i+1), . . . , V.sub.B (0), . . . , V.sub.B (i), V.sub.B (i+1), . . . , where two vertical lines are activated for each of V.sub.A (0), etc.
An example of the interlace circuit 104 and the buffer circuit 105 is illustrated in FIG. 3 wherein there is shown a single interlacing section 30 and a single buffer section 35. The interlace circuit 104 and the buffer circuit 105 are, of course, composed of a plurality of such sections; however, because each section is identical, only single sections are shown here. An output signal VS(i) from the vertical shift register 103 is applied to drains of each of four FET devices 91-94 of the interlacing section 30. The signal FA (Frame A) is applied to the gates of the devices 91 and 92, while the signal FB (Frame B) is applied to the gates of the devices 93 and 94. The conductive channels of the devices 92 and 93 are connected in parallel. The reason for this is the necessity for causing overlapping activation of pairs of vertical lines for interlaced scanning. The sources of the devices 91 and 94 and the paired devices 92 and 93 are coupled to gates of corresponding driver devices 95 in the buffer section 35. The drains of the devices 95 are coupled in common to a source of the clock signal .phi..sub.V3. "Bootstrap" capacitors 98 are coupled between the gates and drains of the devices 95. The .phi..sub.V2 clock signal is applied through devices 99 to discharge the vertical electrodes.
As should be clear by this point, the above-described prior art approach is disadvantageous because of the necessity for simultaneously activating two of the vertical lines during the scanning of the sensor array. Because two lines must always be simultaneously activated, a high drive capability for the buffer circuit is required. Also, relatively complex interlacing and buffer circuits are necessary.
Accordingly, it is a primary object of the present invention to obviate the above-mentioned drawbacks of the prior art approach discussed above, specifically, to provide a solid-state color imager in which it is necessary to activate only a single vertical line at a time for scanning the sensor array, and accordingly to provide an imager which requires only very simple and physically small vertical interlacing and buffering circuitry.